Gas lasers and, in particular, helium-neon (HeNe) lasers are well known in the art. See, for example, the article by Bhogi Patel entitled "The Helium-Neon Laser: What It Is and How It Works" published in the January 1983 issue of PHOTONICS SPECTRA, pp. 33-38. Typically, in such devices, a high d.c. voltage, in the range between 5 KV and 10 KV, is applied across the tube and its associated current-limiting series resistor in order to break down the gas and establish a steady-state discharge at a normal operating current of a few milliamperes. The process is set in motion when an initiating charged particle (most often an electron present within the gas volume between the tube electrodes and subject to the electric field established by the applied voltage) gains sufficient energy to produce positive ion-electron pairs by collision with neutral gas atoms. The secondary electrons thus created are, in turn, accelerated by the electric field to produce additional ion-electron pairs, resulting in an exponential growth in the number of charged particles moving in the direction of the electric field. Breakdown, and a transition to the lower voltage, self-sustaining glow discharge mode occurs when the positive ions formed move to the cathode and produce enough electrons by secondary emission to replace the initiating electron.
The time delay between the application of the high voltage to the laser tube and the appearance of a glow discharge is termed the breakdown delay. This delay has two components. One component, the statistical delay, is associated with the time required for an ionizing particle to appear at an appropriate place in the tube to initiate breakdown. The other component, termed the formative delay, is the time required for the initial electron avalanche to build up to the point where a glow discharge appears. This latter delay, at the large voltages typically used to start gas lasers, is relatively small compared to the statistical delay and can be neglected for most practical applications. The statistical delay, on the other hand, can be significant. Techniques for reducing this delay are described, for example, in U.S. Pat. Nos. 3,792,372; 4,190,810; and 4,352,185. The first patent discloses the use of an additional starting electrode. The second adds a strip of electrically conductive plastic material on the outer lateral surfaces of the laser tube. Such techniques, even if efficacious, tend to add to the cost of the laser and, hence, are not commercially attractive. In the last of the above-identified patents, a small amounts of radioactive material is included within the tube. However, restrictive BRH regulations regarding the use, labeling and disposal of radioactive materials tend to discourage their use.
It is also well known that gas breakdown can be accelerated in tubes exposed to light. However, in practice, laser tubes are generally enclosed in protective housings and, as such, are in total darkness prior to the initiation of a discharge.
Absent such ionizing mechanisms, the statistical delay is determined by the need to wait for a cosmic ray event, or some other radioactive event to occur that generates at least one electron in a favorable position between the tube electrodes. In commercially manufactured tubes of the type described in the above-identified article, and to be described in greater detail hereinbelow, delays of the order of one to more than 15 seconds are common. For many applications, such as bar code scanners commonly use in supermarkets, delays of such magnitude are unacceptable.
It is, accordingly, the broad object of the present invention to reduce the breakdown delay in cold cathode tubes such as gas lasers.
A second object of the invention is to reduce the magnitude of the high voltage necessary to produce breakdown in gas lasers without adversely affecting the breakdown delay.